The present invention relates to a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium which has excellent in the magnetic characteristics and reliability.
In order to meet the recent demand for a high recording density in a magnetic recording medium, especially a magnetic disk, development of a magnetic disk comprising a thin film magnetic medium has been made. A general structure of the magnetic disk is shown in FIG. 1. An underlayer 2 composed of a nonmagnetic material is formed on a substrate 1 composed of a nonmagnetic material such as aluminum, and a magnetic medium 3 and a protective film 4 are formed on the underlayer 2. Generally, the underlayer is an amorphous nickel-phosphorus alloy layer formed by electroless plating. As the magnetic medium, an iron oxide (.gamma.-Fe.sub.2 O.sub.3) layer formed by sputtering or a cobalt-phosphorus alloy layer formed by plating is used.
It is required that the underlayer should be nonmagnetic, should have a mechanical strength sufficient to resist the impingement between the magnetic head and the disk and should have a sufficient corrosion resistance in the environment where the disk is used. In connection with the nonmagnetic characteristic, it is important that the underlayer should not be magnetized by heating effected in the magnetic disk-forming step. As the temperature of magnetization, i.e. the temperature at which an amorphous alloy crystallizes to exhibit ferromagnetic properties, of the underlayer is elevated, it becomes possible to elevate the temperature for forming the magnetic medium and the protective layer, and therefore, a magnetic medium having further improved magnetic characteristics and a protective layer having further enhanced corrosion resistance and abrasion resistance can be obtained.
A nickel-phosphorus alloy ordinarily used for the underlayer is amorphous and nonmagnetic in an as-plated state, but if the alloy is heated, the alloy crystallizes at about 250.degree. C. and is separated into nickel and a nickel-phosphorus compound (Ni.sub.3 P), with the result that the underlayer is magnetized by nickel which is a ferromagnetic material. Accordingly, in case of an underlayer composed of an amorphous nickel-phosphorus alloy, the temperature for forming the magnetic medium and the protective layer should be lower than 250.degree. C., thereby making it difficult to impart excellent characteristics to the magnetic medium and the protecting layer. Moreover, the amorphous nickel-phosphorus alloy layer is insufficient in mechanical characteristics and corrosion resistance. More specifically, with an increase of the recording density, the flying height of the magnetic head is reduced and hence, the frequency of the impingement between the magnetic head and the disk is increased. In the case of the nickel-phosphorus alloy, since the tensile strength and hardness are low, the magnetic disk is readily damaged by the impingement. Furthermore, since the bit length as the recording unit is shortened with the increase of the recording density, even a very small defect owing to corrosion causes an error. Since the corrosion resistance of the nickel-phosphorus alloy is poor, the corrosion propagates even to the magnetic medium and the error is increased, resulting in reduction of the reliability of the disk.
As the prior art concerning the underlayer of a magnetic disk, there can be mentioned Japanese Patent Application Laid-Open Specification No. 33900/76. It is taught therein that addition of zinc or manganese to a nickel-phosphorus alloy is effective in improving the coercive force and the magnetic remanence of a magnetic medium formed on the underlayer. However, according to this conventional technique, no improvement is attained in the temperature of magnetization, mechanical characteristics or corrosion resistance.